It is known that a rotating member, such as a phonograph turntable, driven by an electric motor by way of a transmission means or coupling member which may be subject to slip because it does not provide a fully positive transmission of the drive force, for example a rubber belt or a friction drive wheel, tends to suffer from a drop in its speed of rotation, when subjected to a loading, in relation to its operating condition when the rotating member is not loaded or in relation to the speed of rotation of the motor, which is assumed to be at least substantially constant. This fall in rotary speed, which is produced by the load on the rotating member causing the transmission means to slip, while the speed of rotation of the motor remains constant, is a phenomenon encountered with any transmission means which is liable to slip, and the slippage is dependent on the load on the rotating member, although on the nature of the transmission means coupling the drive motor to the rotating member. It is thus found that the degree of slip suffered by the transmission means increases in proportion to the increase in the loading on the rotating member. The result of this slip, when it occurs, is that the ratio between the speed of rotation of the driven rotating member and the speed of the drive motor is reduced.
It will be appreciated that this phenomenon is undesirable in many cases of practical operations, more particularly for example when the speed of rotation of the rotating member is required to remain within close limits, as in the case of the turntable of a phonograph or gramophone, or a capstan in a magnetic tape device, for example a tape recorder and/or reproducer. However in many cases it is essential that the transmission means or coupling member between the drive motor and the rotating member driven thereby is resilient, with the result that the transmission is inherently liable to suffer from slip when the rotating member is loaded. Consequently serious difficulties are encountered in attempting to maintain a constant speed of rotation of the rotating member when the rotating member is subjected to varying loadings, while the speed of rotation of the motor driving the rotating member remains constant. To revert to the above-stated example of a belt-driven phonograph turntable, a fluctuating turntable loading will usually occur when for example a disc cleaning device is laid on or removed from the disc on the turntable. In this case, the loading on and thus the speed of rotation of the turntable will vary resulting in poor sound reproduction.